Gomelsky, M, PI SUMMARY Listeria monocytogenes is one of the most problematic causative agents of foodborne illness. Despite a relatively low number of cases, listeriosis is ranked as the deadliest and the fourth costliest foodborne disease in the USA among all diseases caused by bacterial, parasitic and viral foodborne pathogens. We have characterized a major regulatory system involving the second messenger, c-di-GMP, in L. monocytogenes and found that it plays unexpectedly important roles at the interface between the saprophitic and the pathogenic lifestyles of this bacterium. One of the consequences of elevated intracellular c-di-GMP levels is severe inhibition of mammalian cell invasion. This project is aimed at (i) deciphering the mechanism of c-di-GMP- dependent inhibition of cell invasion, which appears to involve key regulators of biosynthesis and virulence, and (ii) identifying natural compounds that strongly affect intracellular c-di-GMP levels in L. monocytogenes. This project is expected to shed light onto the interplay between c-di-GMP signaling, metabolism and virulence in L. monocytogenes that is emerging as a novel regulatory paradigm. Results of this project may also lead to identifying natural compounds that inhibit listerial invasiveness and therefore may be used to reduce frequency of listerial infections.